In a medical field, doctors display medical images of patients on monitors and interpret (read) the displayed medical images to observe the state of lesions or the change of the lesions over time. Examples of apparatuses configured to generate such medical images include simple X-ray imaging apparatuses, X-ray compute tomography (X-ray CT) apparatuses, magnetic resonance imaging (MRI) apparatuses, nuclear medicine diagnosis apparatuses (such as SPECT and PET), and ultrasound diagnostic imaging apparatuses (ultrasonography (US)).
For example, in the mammary gland, diagnostic imaging may be performed through the procedure of identifying the position of a breast lesion on an image captured with MRI and then observing the state of the lesion using an ultrasound diagnostic imaging apparatus. Here, in a typical radiographic protocol in the mammary gland, generally, MRI is performed in a prone position (the position of the body lying face down), and ultrasound imaging is performed in a supine position (the position of the body lying face up). A doctor performs ultrasound imaging of the lesion after estimating the position of a lesion in the supine position from the position of the lesion obtained from an MRI image in the prone position while taking into account the deformation of the breast caused by the difference in the position of the body during the imaging.
However, the deformation of the breast caused by the difference in the position of the body during the imaging may be large so that the position of the lesion estimated by the doctor may be deviated from the actual location. Therefore, the extraction of an ultrasound image of the lesion the doctor wishes to observe may fail, or a long time may be required to find the lesion. This difficulty may be overcome by performing MRI in the supine position which is the same as the position of the body during ultrasound imaging. However, imaging in the supine position may be affected by the breathing of the subject being examined, and another difficulty may arise in that a sharp MRI image necessary for interpretation will not be obtained.
If an MRI image obtained by imaging in the prone position is deformed through image processing and a virtual MRI image which is obtained by imaging in the supine position is successfully generated, the position of the lesion is identified from the deformed MRI image, and therefore ultrasound imaging of the lesion can be realized without attention paid to the difference in the position of the body during the imaging. For example, after an MRI image obtained by imaging in the prone position is interpreted and the position of the lesion on the image is obtained, the position of the lesion on a virtual MRI image in the supine position can be calculated based on information regarding posture change from the prone position to the supine position. Alternatively, the generated virtual MRI image in the supine position may be interpreted and therefore the position of the lesion on this image can directly be determined.
To achieve this, with the use of a method disclosed in NPL 1, an MRI image in the prone position can be deformed to have the same shape as an MRI image in the supine position. In the disclosed method, first, a virtual MRI image in the supine position is generated from an MRI image in the prone position using a physical simulation. Then, deformation registration between the virtual MRI image in the supine position and an actual MRI image obtained by imaging in the supine position is executed based on the similarity of pixel values. Based on the correspondences obtained in the above process, the process of deforming the MRI image in the prone position to have the same shape as an MRI image in the supine position is executed.
NPL 2 discloses a technique for obtaining a statistical motion model (SMM) by obtaining in advance, using a physical simulation, a deformed shape group for various settings of a parameter regarding the deformation (hereinafter referred to as a deformation parameter) of a target object and by applying principal component analysis to the result. NPL 2 also discloses a technique for associating shapes acquired before and after deformation by comparing shape data acquired after deformation, which is obtained separately, with the shape of the surface portion of the SMM and by estimating deformation.
In order to correctly perform the process using the method described in NPL 1, it is necessary to obtain in advance an accurate value of the deformation parameter of the target object. That is, if the deformation parameter is not obtained, it is difficult to apply the method described in NPL 1. When the deformation parameter is unknown, an approach of attempts to apply a deformation based on all the patterns of the change of the deformation parameter may be conceivable. However, many attempts of deformation may require a large amount of time.
In the method described in NPL 2, deformation is estimated only using the outline shape of the target object, leading to ambiguous estimation of deformation on a smooth curve such as the surface of the human breast. Thus, high-accuracy estimation of deformation may not be feasible.